The differentiation of murine thymocyte subsets has been further examined. Intravenous (i.v.) and intrathymic (i.t.) cell transfers into irradiated congenic mice, previously had demonstrated a precursor subset of dull Ly-l+ (dLyl) thymocytes lacking CD4 and CD8, i.e. double negative (DN). These dLyl cells contain committed thymocyte progenitors with limited capacity for generation of other hematopoietic cells. Additional, more immature thymocyte precursors have been found in bone marrow by i.v. and i.t. transfer, but i.t. transfers bypass the need for "homing" of cells to the thymus. This i.t. methodology has enabled us to transfer the CD4+,CD8+, double positive (DP) blasts, and thymocytes expressing only CD4+ or CD8+. The DP blasts differentiate almost exclusively to CD4+ cells, whereas dLyl cells first yield DP cells, followed by CD4+ and then CD8+ cells. Isolated CD4+ or CD8+ cells are incapable of extensive growth or repopulation of other subsets and isolated NK cells do not become thymocytes. We have also determined that limited pretreatment with estradiol, without irradiation, renders the recipient's thymus transiently receptive for thymocyte precursors. Exogenous estradiol administration selectively depletes a subcapsular thymic blast population, which permits limited thymic repopulation by donor-derived dLyl or bone marrow cells. However, studies of bone marrow and thymus precursors, examined at multiple time points in standard irradiation chimeras, indicate that although there are marked differences between these two types of precursors, there is no definitive subcapsular localization. The phenotype of rat and mouse DN thymocytes has been compared. The majority of both rat and mouse DN cells are proliferating when first isolated and have low CD5 (Ly-l) and high Thy-l expression. However in remarkable contrast to the mouse, the rat DN thymocytes reveal no interleukin 2 receptor expression. Finally, continuing studies examining oncogene expression in normal mouse lymphocyte subsets has revealed a selective increase in ets oncogene expression in CD4+ thymocytes compared to other thymic subsets or CD4+ lymph node T cells.